FIGS. 8(a) and 8(b) are views of a semiconductor laser 80 including a window structure of the facets of the laser. FIG. 8(a) is an end view showing one of the facets of the semiconductor laser and FIG. 8(b) is a longitudinal sectional view perpendicular to the facets taken along line 8b--8b of FIG. 8(a). The semiconductor laser 80 includes an n-type GaAs substrate 1 on which are successively disposed an n-type Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 2 and an active layer 3 having a multiple quantum well structure including a plurality of alternating AlGaAs well layers and AlGaAs barrier layers. Successively disposed on the active layer 3 are a p-type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 4, an n-type GaAs current blocking layer 8, and a p-type GaAs second contacting layer 9. The upper cladding layer 4 includes a portion sandwiched between the current blocking layer 8 and the active layer 3 and a centrally disposed ridge 14. Centrally disposed within the laser, sandwiched by parts of the current blocking layer 8 and contacting the ridge 14 of the upper cladding layer 4, is a p-type GaAs first contacting layer 5 that is part of the ridge 14. The first contacting layer 5 contacts the second contacting layer 9 in the same plane as the interface of the second contacting layer 9 and the current blocking layer 8.
The semiconductor laser of FIGS. 8(a) and 8(b) includes a window structure 6 at each of its facets, best seen in FIG. 8(b). The window structures 6 comprise regions into which silicon ions have been implanted. The implantation, after annealing, causes disordering of the active layer 3 within the region 6 so that regions 7 of the active layer lack the multiple quantum well structure that is present elsewhere in the active layer 3. Electrodes 10 and 11 are respectively disposed on the second contacting layer 9 and the substrate 1.
A method of making the semiconductor laser of FIGS. 8(a) and 8(b) is illustrated in FIGS. 9(a)-9(f). Initially, as illustrated in FIG. 9(a), the lower cladding layer 2, the active layer 3, the upper cladding layer 4, and the first contacting layer 5 are successively and epitaxially grown on the substrate 1. Thereafter, a photoresist film 12 is formed on the first contacting layer 5 and a window 13 is opened in the photoresist film 12 using photolithographic techniques. The opening 13 may be a square 40 microns on a side. The resist film 12 is used as an ion implantation mask and dopant impurity ions, preferably silicon, are implanted in the structure through the opening 13. The photoresist film 12 prevents silicon ions from entering parts of the layers not exposed by the opening 13. The ion implantation step produces an implanted region 6 illustrated in a cross-sectional view in FIG. 9(c) that extends through a part of the first contacting, upper cladding, and active layers and into the lower cladding layer 2. After removal of the photoresist film 12, the implanted silicon ions are activated by annealing, resulting in disordering of the multiple quantum well structure in the portion of the active layer 3 where the silicon ions are present. This annealing and activation process is conventionally carried out in an As ambient at a temperature of at least 700.degree. C.
As illustrated in FIG. 9(d), an etching mask of an electrically insulating material, such as Si.sub.3 N.sub.4, SiO.sub.2, or the like, is formed on a part of the first contacting layer 5 in a stripe-shaped pattern. That insulating film mask is used to define a ridge extending between the facets of the semiconductor laser and aligned with the implanted and disordered regions of the active layer 3. Where not protected by the etching mask 15, the first contacting layer 5 and the upper cladding layer 4 are chemically etched and removed, leaving a ridge structure 14, as shown in FIG. 9(d), in place. The selective etching may employ as an etchant a mixture of tartaric acid and hydrogen peroxide or a mixture of sulfuric acid, hydrogen peroxide, and water.
As illustrated in FIG. 9(e), the n-type GaAs current blocking layer 8 is epitaxially grown on the upper cladding layer 4 opposite the active layer 3 and in contact and sandwiching the ridge 14. The insulating film etching mask 15 remains in place during the growth step. If a chemical vapor deposition process, such as metal organic chemical vapor deposition (MOCVD), is employed to grow the current blocking layer 8, no crystalline growth occurs on the insulating film mask 15. Following the growth of the current blocking layer 8, the insulating film mask 15 is removed by wet or dry etching and the second contacting layer 9 is grown on the current blocking layer 8 and the first contacting layer 5. The semiconductor laser is completed by forming the electrodes 10 and 11. Although not illustrated, but known to those of skill in the art, the process of forming the semiconductor laser includes cleaving to form the facets. FIG. 9(b) illustrates ion implantation to form one window region of many such regions on a semiconductor wafer from which many semiconductor lasers are obtained. When the wafer is cleaved at locations intersecting the windows 13, each of the resulting semiconductor lasers includes two opposed facets, as shown in FIG. 8(b). Only one of those facets is illustrated in the end view of FIGS. 8(a) and 9(f).
When the semiconductor laser of FIGS. 8(a) and 8(b) is forward biased, holes are injected into the active layer 3 including the quantum well structure through the second contacting layer 9, the first contacting layer 5, and the upper cladding layer 4. Electrons are injected into that active layer 3 through the substrate 1 and the lower cladding layer 2. The electrons and holes recombine in the active layer 3 and produce light. When the current flow exceeds the laser oscillation threshold of the semiconductor laser, laser oscillation producing coherent light occurs. The flow of holes into the active layer 3 is concentrated in a central portion of that layer by the ridge 14 that is confined by the current blocking layer 8. The rectifying junctions formed between the current blocking layer 8 and the ridge 14 restrict the area of current flow, increasing the current density for producing laser oscillation. The GaAs current blocking layer 8 has a smaller energy band gap than the effective energy band gap of the active layer and, therefore, absorbs light produced in the active layer 3. This light absorption concentrates the light produced by the laser in the ridge 14, producing a stable, single mode oscillation.
The window structure 6 enables the light output power to be increased without risk of damage to the semiconductor laser at the facets. In semiconductor lasers employed to retrieve stored information from a compact disc, the laser light has a wavelength of about 800 nanometers and the maximum power is limited by heating at the facets which, in a worst case, causes melting of the semiconductor materials in the laser and destruction of the laser. Heating occurs at the facets because of the absorption of light. In order to increase the output power of the laser light that may be safely produced without risking damage to the laser, it is necessary to reduce the amount of light absorbed at the facets. The window structure 6 and, particularly, the disordered region 7 of the active layer 3 at each of the laser facets reduce light absorption. The disordering of the multiple quantum well structure within the active layer 3 at the facets increases the effective energy band gap of the active layer in those regions, resulting in reduced light absorption.
The disordering of the multiple quantum well structure at the laser facets may be easily understood in conjunction with FIGS. 10(a) and 10(b). FIG. 10(a) is a profile of the relative concentration of aluminum in layers of the semiconductor laser. Within the active layer 3, a periodic structure of the well and barrier layers is present. That periodic structure has a periodic variation in the concentration of aluminum, as illustrated in FIG. 10(a). Within the multiple quantum well structure, the aluminum concentration varies from a maximum aluminum concentration of Al.sub.1 to a minimum concentration of Al.sub.2. As seen in FIG. 10(a), the multiple quantum well structure is sandwiched by layers of constant aluminum concentration within the active layer. Still higher levels of aluminum concentration are present in the cladding layers that sandwich the active layer.
FIG. 10(b) illustrates the variation in aluminum composition in the active layer 3 within the window region, i.e., in region 7. Because of the disordering by the implantation of silicon ions and annealing, the aluminum composition within the active layer is essentially homogeneous at a concentration of Al.sub.3, a concentration intermediate the concentrations Al.sub.1 and Al.sub.2, the respective concentrations in a well layer and a barrier layer before disordering. In general, the well layer thickness is no more than 20 nanometers so that disordering of a region of the quantum well structure produced by the diffusion of dopant impurities, such as silicon and zinc, homogenizes the structure, producing a material having an energy band gap larger than the effective energy band gap of the quantum well structure. Thus, the window region 7 of the active layer 3 has significantly reduced absorption of the light produced elsewhere in the active layer 3.
In the ion implantation process illustrated in FIGS. 9(b) and 9(c), the active layer 3 is usually spaced from the exposed surface of the first contacting layer 5 by a distance of at least 1.7 microns, the sum of the thicknesses of the upper cladding layer 4, at least 1.5 microns, and of the first contacting layer 5, a thickness of at least 0.2 micron. These thicknesses are important to ensure that the light produced in the active layer 3 is absorbed within the semiconductor laser structure transverse to the ridge. In order to implant silicon ions through the opening 13 that reach the active layer 3 in the necessary concentration for subsequent disordering of window regions of the active layer 3, the silicon ions must have an energy of at least 2 MeV. These high energy dopant ions damage the compound semiconductor layer through which they pass, producing many crystalline defects. Even though some of the crystalline defects are removed by the annealing step in the activation of the implanted ions, a significant concentration of crystalline defects remains. The crystalline defects absorb some of the laser light and, therefore, interfere with the desired properties, i.e., non-absorption, of the window structure. In addition, the crystalline defects trap the implanted dopant ions and interfere with or prevent their diffusion during the annealing and activation step, limiting the degree of disordering that is achieved in the window structure.